1. Field of the Invention
The present invention relates to a sealing composition for sealing a solar cell, which excels in heat resistance, impact resistance, and long reliability. The present invention also relates a solar cell module in which said sealing composition is used and a building material-integral type solar cell module in which said sealing composition is used.
2. Related Background Art
In recent years, an uprush of consciousness for the problems of pollution has been spread in the world. Among others, a sense of crisis about heating of the earth because of the so-called green house effect due to an increase of atmospheric CO2 has been growing among the people. Therefore, there is an increased societal demand for early realization of a power generation system capable of providing clean energy without causing CO2 build-up.
There have been various proposals which are expected to meet such societal demand. Among those proposals, solar cells are expected to be suitable as an alternative source of energy since they supply electric power without causing such problems as above mentioned, and they are safe and can be readily handled.
Such solar cell includes single crystal silicon solar cells, polycrystal silicon solar cells, amorphous silicon solar cells (including microcrystal silicon solar cells), copper-indium-selenide solar cells, and compound semiconductor solar cells. Of these solar cells, various studies have been made on so-called thin film crystal solar cells, compound semiconductor solar cells and amorphous silicon solar cells since their semiconductor active layers can be relatively easily formed in a large area and in a desired form and they can be easily produced at a relatively low production cost.
Now, in the case of producing a solar cell module using any of these solar cell, it is necessary to protect the solar cell by covering the light incident side by a transparent covering material. As a typical method of producing a solar cell module in this way, it is known that a given solar cell is sealed by a sealing filler material comprising a transparent thermoplastic resin and the outermost surface side of the solar cell is covered by a surface covering material comprising a glass plate or a transparent film comprising a fluororesin or a fluororesin paint. In many cases, a glass plate is used as the surface covering material. A main reason why the glass plate is used at the outermost surface side is that the solar cell module is made to excel in weatherability and scratch resistance so that the photoelectric conversion efficiency thereof is prevented from being reduced due to a reduction in the light transmittance of the surface covering material when the surface covering material is deteriorated. Particularly in view of mechanically protecting the solar cell in the solar cell module, it can be said that the glass plate is one of the most appropriate materials to be used as the surface covering material. However, when the glass plate is used as the surface covering material, there are disadvantages such that the surface covering material is heavy, is difficult to bend, is inferior in shock resistance, and is relatively costly. Especially, when the glass plate is used as the surface covering material, it gives a serious influence to the creep resistance characteristic of the sealing filler material of the solar cell in the solar cell module, as will be described later.
On the other hand, when a flouroresin material is used as the surface covering material, there are advantages such that the surface covering material comprising the fluororesin material excels in weatherability and water-repellency, it is able to diminish a reduction in the photoelectric conversion efficiency of the solar cell module because the fluororesin material as the surface covering material is difficult to deteriorate or stain and therefore, the light transmittance thereof is difficult to decrease, and in addition, the fluororesin material excels in flexibility and is lighter than the glass plate and this situation makes it possible to obtain a solar cell module which is lightweight and excels in flexibility.
As an adhesive in order to bond such surface covering material with the solar cell in the production of a solar cell module, a sealing filler resin material which seals and protects the solar cell is used. As such filler resin material, various transparent resin compositions comprising a transparent thermoplastic resin are selectively used. Such transparent thermoplastic resin can be readily acquired at a reasonable cost and therefore, it can be used in a large amount in order to seal and protect the solar cell in the solar cell module. Specifically, as such transparent thermoplastic resin, ethylene-vinyl acetate copolymer (EVA) or polyvinyl butyral (PVB) is generally used.
However, such transparent thermoplastic resin has disadvantages such that when the solar cell module is exposed to sunlight outdoors over a long period of time (for example, over 20 years), the transparent thermoplastic resin as the sealing filler resin material of the solar cell is liable to partially gel due to continuous irradiation of ultraviolet rays, resulting in it being clouded or it is liable to yellow due to an increase in the number of conjugated double bonds in the chemical structure of the resin as a result of continuous irradiation of ultraviolet rays. The occurrence of such cloudiness or yellowing makes the transparent thermoplastic resin to be poor in the light transmittance, resulting in reducing the photoelectric conversion efficiency of the solar cell module. Especially, when the transparent thermoplastic resin comprises EVA and the solar cell module is integrated with the roof of a building and it is continuously used in sever outdoor atmosphere because the temperature of the solar cell module is elevated to a high temperature of higher than 70xc2x0 C., where the EVA as the transparent thermoplastic resin of the sealing filler resin material is markedly yellowed to cause a serious problem for the photoelectric conversion efficiency of the solar cell module.
Incidentally, the organic resin such as EVA or PVB which is used as the sealing filler resin material is thermoplastic in any case, and because of this, under use condition in outdoors where the surface temperature of the solar cell module is elevated to a high temperature and as a result, the organic resin is softened to suffer creeping where the adhesion of the organic resin is diminished to often generate a so-called microdelamination phenomenon of causing peeling between the sealing filler resin material and the solar cell or the surface covering material. Further in this case, when the surface covering material comprises a resin film, a problem is liable to occur in that the resin film is softened so as to cover the solar cell and because of this, the scratch resistance of the surface covering material is diminished.
In order to solve such problems as above described, there is a proposal in that a crosslinking agent is incorporated into the sealing filler resin material comprising such transparent thermoplastic resin and the transparent thermoplastic resin of the sealing filler resin material is crosslinked by the crosslinking agent. Particularly, for instance, Japanese Patent Publication No. 60579/1983 discloses a filling adhesive sheet for a solar cell, comprising an ethylene series copolymer resin incorporated with a silane coupling agent and an organic peroxide as a crosslinking agent. In this document, it is also described that as the ethylene series copolymer resin, EVA having a vinyl acetate content of preferably less than about 40 wt. %, more preferable 20 to 40 wt. % is preferred.
However, although the EVA disclosed in the above document is good in terms of the adhesion to a glass or a fluororesin series film and it excels in flexibility, improvement of the creep resistance and the scratch resistance is not sufficiently enough. Especially for the scratch resistance of the sealing filler material, it is not sufficiently improved only by way of such heat crosslinking.
In order to improve this situation, there is known a method in which an appropriate reinforcing member comprising a nonwoven glass fiber is contained in the sealing filler material. However, such nonwoven glass fiber is usually impregnated with a binder resin. This binder resin is liable to largely contribute to making the sealing filler material to be yellowed.
In comparison with the weatherability of the thermoplastic resin as the sealing filler resin material, that of the binder resin of the nonwoven glass fiber is not so high, and the binder resin does not contain an additive capable of improving the weatherability. Therefore, the binder resin is more liable to deteriorate in comparison with the thermoplastic resin. Further, the binder resin is distinguished from the thermoplastic resin in terms of the resin kind such that they are not compatible with each other and because of this, when moisture enters in the interface between them, the deterioration of the binder resin is liable to more progress.
For instance, in the case where EVA is used as the thermoplastic resin and a polyvinyl alcohol resin is used as the binder resin of the nonwoven glass fiber, as disclosed in Japanese Unexamined Patent Publication No. 112549/1998, it is known that in accelerated weathering test such as EMMAQUA test, weatherOmeter test, strong black light test, or 150xc2x0 C. heat resistance deterioration test, yellowing chiefly due to deterioration of the binder resin is occurred. Similarly, it is also known that yellowing is occurred also when a hard resin thin layer chiefly comprising an acrylic resin is provided between the surface covering material and the solar cell (or the photovoltaic element) in order to improve the scratch resistance of the solar cell module.
The present is aimed at solving the foregoing problems of the sealing material of the solar cell in the solar cell module in the prior art.
Another object of the present invention is to provide a sealing composition for a solar cell, having an improved heat resistance, an improved creep resistance, and an improved scratch resistance.
A further object of the present invention is to provide a highly reliable solar cell module in which said sealing composition is used.
A further object of the present invention is to provide a highly reliable building material-integral type solar cell module in which said sealing composition is used.
As a result of experimental studies by the present inventors in order to solve the foregoing problems in the prior art and also in order to achieve the above objects, the present inventors obtained a finding that a scaling composition as will be described below is the most appropriate in order to desirably solve the problems in the prior art.
That is, the present invention provides a sealing composition for a solar cell, comprising a polymer blend or a polymer alloy comprising at least one kind of a polyolefin series copolymer and at least one kind of a crystalline polyolefin.
It is preferred for the polyolefin series copolymer and the crystalline polyolefin to have a relationship such that the polyolefin series copolymer has a fusing point Ta by differential scanning calorimetry (xe2x80x9cDSCxe2x80x9d) method, the crystalline polyolefin has a fusing point Tb by DSC method, and the fusing point Tb is greater than the fusing point Ta. Particularly, it is preferred that the fusing point Ta of the polyolefin series copolymer is in a range of 50 to 110xc2x0 C., and the fusing point Tb of the crystalline polyolefin is in a range of from 110 to 170xc2x0 C.
Specific preferable examples of the crystalline polyolefin are polyethylene, polypropylene, ethylene-propylene copolymer, ethylene series copolymer, and propylene series copolymer. The crystalline polyolefin may comprise one or more kinds of polymers selected from these.
Specific preferable examples of the polyolefin series copolymer are ethylene-vinyl acetate copolymer, ethylene-acrylate ester copolymer, ethylene-methacrylate ester copolymer, ethylene-acrylic acid copolymer, ethylene-methacrylic acid copolymer, ethylene-acrylate ester multi-copolymer, ethylene-methacrylate ester multi-copolymer, ethylene-acrylic acid multi-copolymer, and ethylene-methacrylic acid multi-copolymer.
The polyolefin series copolymer may comprise one or more kinds of copolymers selected from these.
Each of the polymer blend and the polymer alloy is preferred to have a total light transmittance of more than 80% in a wavelength region of 400 to 1000 nm at a thickness of 0.5 mm for the polymer blend or the polymer alloy when an air is made to be a reference.
In addition, each of the polymer blend and the polymer alloy is preferred to have a melt flow rate measured by a method of ASTM D-1238 as of the end of 1999 at a temperature of 190xc2x0 C. and at a load of 2.16 Kg. which is in a range of from 0.1 g/10 min to 40 g/10 min.
The present invention provides a solar cell module comprising a photovoltaic element (a solar cell) arranged between a surface covering material and a back face covering material, said photovoltaic element being sealed by a sealing material between said surface covering material and said back face covering material, characterized in that said sealing material comprises aforesaid sealing composition.
The present invention provides a building material-integral type solar cell module comprising a photovoltaic element (a solar cell) arranged between a surface covering material and a back face covering material, said photovoltaic element being sealed by a sealing material between said surface covering material and said back face covering material, characterized in that said sealing material comprises aforesaid sealing composition.